Natural gas may be liquefied by employing so-called reverse-Brayton cycles (sometimes called gas recycle or nitrogen recycle), where isentropically expanding gaseous refrigerant is used to provide the refrigeration. A natural gas feed is typically at a higher pressure than a nitrogen refrigerant stream used to cool it. It is conceivable, therefore, that natural gas may leak into a nitrogen refrigerant circuit within a liquefier heat exchanger. For example, in a plate-and-fin heat exchanger, a parting sheet may leak allowing a natural gas stream to enter the refrigerant cycle. In a wound-coil heat exchanger, for example, a tube may leak allowing a natural gas stream to enter the refrigerant cycle in the shell portion of the exchanger. In either case, hydrocarbons, and methane in particular, may build up in the refrigerant circuit lowering the cycle's efficiency. The cycle efficiency will be lowered because the system pressure must be lowered to keep the refrigerant near dew point at the cold expander discharge. The system pressure must be lowered to avoid excess liquid at the discharge of the expander that may cause damage to the equipment. Even small leaks can build up over time. One of the advantages of using pure nitrogen, for example, as a refrigerant is that it is inert, therefore, a hydrocarbon leak into the inert refrigerant stream would then make it flammable.
One method for dealing with a hydrocarbon leak in a refrigerant circuit required lowering the feed gas pressure, which in turn, lowered or even reversed the leak in the refrigeration circuit. Lowering the feed gas pressure, however, lowered the cycle's efficiency. If the liquefaction and subcooling took place in separate heat exchangers, for example, and the leak occurred in the subcooler, then the leak could also be mitigated by lowering the pressure of the liquefied natural gas (LNG) entering the subcooler to slightly below the nitrogen pressure with no effect on the cycle's efficiency.
Another method used to deal with a relatively small leak was to purge the refrigerant circuit and increase the pure nitrogen makeup. A small makeup is normally required to compensate for compressor seal and other losses. Purging, however, wastes nitrogen, the principal component of the gaseous refrigerant. The purge could also be combined with the fuel, but doing so would increase the fuel's nitrogen content thereby causing more nitrogen oxide to be released into the air. Further, the nitrogen makeup, or the ability to regenerate the nitrogen for the refrigeration cycle, may be limited on a floating application.
Another method disclosed use of natural gas liquefiers, where isentropically expanding gaseous refrigerant was used to provide the refrigeration and a portion of refrigerant was liquefied to reflux a distillation column to remove nitrogen from liquefied natural gas product depending on the feed compositions and liquefied natural gas product specifications. The nitrogen was rejected, however, from the liquefied natural gas product, and not from the gaseous refrigerant.
There is, therefore, a need in the art to address the possible leak problem without purging, without interrupting the production until the next scheduled shutdown when the repairs can be made, and without decreasing the efficiency of the system.